Tracheal intubation is used in respiratory medicine to deliver or remove a fluid to the airways of a patient. Tracheal intubation with an endotracheal tube is commonly used during general anesthesia and when critically ill patients require airway protection and mechanical ventilation. Tracheal tubes include those used in tracheostomies as well as endotracheal tubes. Under certain conditions of long term ventilation, a tracheostomy tube is inserted through a surgical opening through the neck. An endotracheal tube is inserted into the trachea through either the mouth or nose (nasotracheal tube). Tracheal tubes are used for ventilation, and removal of secretions. Typically, endotracheal tubes are disposable plastic tubes, easily placed through the mouth or nose, that guarantee a patent conduit for the delivery of respiratory gases. A seal between the outer wall of the tracheal tube and the inner lining of the trachea (the tracheal mucosa) must be formed. Most endotracheal tubes (except those for small children) provide a very compliant, thin walled inflatable cuff that forms a seal with the proximal tracheal rings. This seal allows for positive pressure ventilation at normal airway pressures with minimal leakage. The seal thus provides a closed circuit for ventilation and also prevents aspiration of pharyngeal contents into the respiratory tract. This seal is usually formed by inflating a pressurized cuff that surrounds the tracheal tube. The pressure in the cuff must be adequate to form a seal, but it is known that over-pressurization will cause tracheal trauma including hemorrhage, ulcers, perforation and strictures. The main cause of trauma is the loss of blood flow (ischemia) and resultant necrosis of the tracheal lining.
Cuff pressures are typically set between 10-50 mm Hg. This range of pressures is relatively wide and for any particular patient the formation of a seal without creating an ischemia will require holding the pressure in a much narrower range. Moreover, because tracheal tubes are used in a dynamic environment, the pressure required to maintain an adequate seal will vary. Finally, cuff pressure will vary due to the diffusion of nitrous oxide into the cuff. Thus, establishing a correct cuff pressure and correctly regulating the cuff to this pressure are both important. For example, during mechanical ventilation, the intracuff pressure must be low enough to allow tracheal capillary perfusion, thus reducing the risk of ischemia, while being high enough to prevent either loss of tidal volume or significant aspiration.
The use of high volume, low pressure endotracheal tube cuffs starting in 1973 reduced the incidence of complications from prolonged tracheal intubation. This more compliant cuff design provides a satisfactory tracheal seal for positive pressure ventilation associated with significantly lower cuff-tracheal mucosa contact pressure. Unfortunately, when lung compliance significantly decreases--as often occurs in critically ill ICU patients--high airway pressures are transmitted through the distal cuff surface during inspiration. Cuff-tracheal mucosal contact pressures approach peak airway pressures as high as 50 mm Hg during each inspiration. This contact pressure far exceeds tracheal mucosal perfusion pressure of 15 mm Hg and significant ischemic damage can occur, and in fact mucosal ischemic damage commonly occurs in observed patient populations. Endoscopic and biopsy studies suggest a high incidence of mucosal ulceration, cartilage infiltration with bacteria, and ultimately, scar formation with the development of clinically significant airway narrowing. Local mucosal defense mechanisms are inhibited during tissue ischemia, allowing bacteria to multiple and invade deeper structures.
An additional cause of cuff pressures that exceed mucosal perfusion pressure is brought about during nitrous oxide general anesthesia. Nitrous oxide diffuses into a cuff eighty times faster than nitrogen diffuses outward, producing a significant increase in cuff volume such that cuff pressures often exceed 60 mm Hg, and the above-described ischemia and deterioration of the mucosa result.
Others have recognized and proposed solutions to the problems associated with excessive cuff pressures during prolonged intubation. For example, U.S. Pat. No. 5,235,973--Levinson discloses a system for monitoring and controlling cuff pressure that requires both an inflation line and a monitoring line connected to the cuff. The monitoring line is used to determine when additional pressure should be applied to the cuff, and is governed by the pressure in the inspiration line of the ventilator. However, this system raises cuff pressure with each inspiration to avoid loss of tidal volume without regard for cuff-mucosal perfusion pressure and thus cannot provide satisfactory pressure regulation. Another pressure control system is disclosed in U.S. Pat. No. 4,924,862--Levinson. In this system, pressure relief valves connect the cuff and a source of pressurized gas. A high pressure relief valve regulates cuff over-pressure, while a low pressure relief valve is controlled by a flow detector monitoring the cuff inflation line. A continuous flow in the cuff inflation line is indicative of a cuff leak. Cuff pressure regulation via mechanical valves is disclosed by U.S. Pat. Nos. 4,770,170 and 4,825,862 both to Sato et al. However, as well known to those of skill in the art, mechanical valves have performed poorly in clinical use.
One pressure regulation device known as the Cardiff Cuff Controller has been reported in the literature. See Morris et al., J. Med. Eng. & Tech., Vol. 9, No. 5 (Sept./Oct. 1985) at pp. 229-30; Cobley et al., "Endobronchial Cuff Pressures," British J. Anesthesia, 70:576-78 (1993). This device regulates cuff pressure using an air reservoir and pump that connect to the cuff via a controlled valve. A relief valve is also provided to alleviate over-pressure conditions. Use of this device is also reported in the literature. See Willis et al., "Tracheal tube cuff pressure: Clinical use of the Cardiff Cuff Controller," Anethaesthesia 43:312-14 (1988); and Cobley et al., referenced immediately above.
As explained above, the prevalence of tracheal tubes, and in particular endotracheal tubes, has led to detrimental effects being observed, particularly when patients are intubated for a long period of time. For example, pulmonary complications are caused by aspiration during prolonged orotracheal intubation. The current use of high volume, low pressure cuffs alleviates this problem to a certain extent. See Spray et al., "Aspiration Pneumonia: Incidence of Aspiration with Endotracheal Tubes," Am. J. Surg. 131:701-03 (June, 1976). However, aspiration can and does occur even using a properly designed high volume, low pressure cuff inflated to and maintained at an appropriate pressure. Regurgitated gastric fluids and pharyngeal secretions that enter the proximal trachea often accumulate above the inflated cuff and may channel to soil the more distal trachea and lungs. Physicians believe this pathway to be the most common mechanism producing hospital acquired bronchitis and pneumonia. As many as twenty-five percent (25%) of patients requiring more than several days of intubation and ventilation within an ICU develop nosocomial pneumonia. Typically, oral and pharyngeal secretions are removed by nursing personnel on an as-needed basis. However, the hand-held suction catheters currently in use fail to drain the subglottic region. As much as 150-200 ml of liquid secretions have been removed over a 24 hour period in one study using once hourly manual syringe evacuation. Significant problems with thick, viscous secretions plagued prior endotracheal tubes that attempted to drain this region. Small bore suction tubing has to be replaced with much larger total volume tubing but not at the expense of greatly increasing the outer diameter of the endotracheal tube at the vocal cord level.
The literature reports a relationship between upper gastrointestinal, oropharyngeal colonization and the occurrence of pneumonia. See Mahul et al. "Prevention of nosocomial pneumonia in intubated patients: respective role of mechanical subglottic secretions drainage and stress ulcer prophylaxis," Intensive Care Med. 18:20-25 (1992). In the Mahul et al. study, both hourly drainage of subglottic secretions and the use of sucralfate or antacids as a prophylactic measure were tried as alternative methods of preventing pneumonia. The drainage of subglottic secretions was performed using a HI-LO EVAC endotracheal tube manufactured by Mallinkrodt; this tube has an elliptic dorsal opening above the cuff that is connected to a separate, integral aspiration lumen. A small bore suction channel was incorporated into the wall of the tube with its distal opening 5.0 mm above the inflated cuff. The antacid cytoprotective agent was administered via a nasogastric tube separate and distinct from the endotracheal tube. It is noted that prior study of this type of endotracheal tube by this inventor's laboratory found a high incidence of suction channel blockage with long term use. In addition, the rate and viscosity of removed secretions is greatly limited by the small lumen.
Thus, in a long term intubation subject, pathogenic bacteria multiply in the pool of secretions that accumulate above the inflated endotracheal tube cuff, providing a significant inoculum when aspirated into the distal trachea. Irrigation and drainage are the methods of choice when attempting to remove infected material from the body. Irrigation dilutes the bacterial colony count and suction drainage removes this material from the body. Applying this technique to the area below the vocal cords and above the inflated cuff (subglottic region), may reduce the incidence and amount of infected secretions that reach the lungs and therefore prevent nosocomial pneumonias. Subglottic irrigation with a bactericidal solution may also significantly decrease the infectivity of secretions that may be aspirated into the distal trachea.
For the foregoing reasons, others have attempted to construct endotracheal tubes that incorporate a suction device to draw fluids from the pharynx or a channel to irrigate a body area. U.S. Pat. No. 2,854,982 discloses a nasopharyngeal tube that comprises an outer tube surrounding a central lumen. The outer tube includes orifices for providing suction above the cuff. U.S. Pat. 3,593,713 discloses a device similar to the aforementioned patent but uses the outer tube to provide irrigation. U.S. Pat. No. 4,468,216 also discloses an irrigation/suction catheter with a central lumen surrounded by an outer tube. The outer tube is used to suction liquids provided via the central lumen.
Despite the above-described attempts of the prior art, however, there are currently no fully satisfactory tracheal tubes that can be left in place for long periods of time. As shown above, it is desirable to regulate the cuff pressure of a tracheal tube. Maintaining such a constant, controlled cuff pressure avoids under-inflation, and the concomitant loss of the mechanical barrier against gross spread of secretions. It is also desirable to avoid over-inflation and thus avoid ischemic mucosal damage that inhibits the local host defenses against infection. It is further desirable to prevent the distal spread of infected subglottic secretions. Toward this end, the subglottic region should be drained and it is further desirable to dilute the bacterial inoculum by irrigation and possibly with bactericidal medication and to frequently remove this material by suction, prior to channeling around the inflated cuff. Such steps will reduce the incidence of conditions such as nosocomial pneumonia and irritation of the mucosa.
It is therefore an object of the present invention to provide methods and apparatus for intubating a patient that simultaneously irrigates the subglottic region with saline or a bactericidal solution, frequently drains the subglottic secretions by controlled suction, and by servoregulating cuff pressure over a very narrow range. It is a further object of the present invention to provide such methods and apparatus in a form that is simple and efficient, and that can be easily manufactured.